1. Field of the Invention
The present invention relates to a color image forming apparatus. More particularly, the invention relates to a color image forming apparatus suitable for use in a laser printer, a digital copying machine, or a multi-function printer, in which a plurality of fluxes emitted from a plurality of light source means scan the corresponding surfaces to be scanned via a single polygonal deflector (polygonal mirror).
2. Description of the Related Art
It is the conventional practice to scan the surfaces of a plurality of photosensitive drums via a single polygonal mirror by means of fluxes emitted from a plurality of light source means, and deflection-scan on two or more different deflection surfaces. The write positions of the surfaces of the photosensitive drums are detected by any of the following two methods.    (1) Japanese Patent Laid-Open No. 5-19586 discloses a method of determining a write position on a photosensitive drum on the basis of retrieval information obtained from write position detecting means; and    (2) Japanese Patent Laid-Open No. 4-313776 shown in FIG. 11 discloses a method of detecting a write position by a single write position detecting means, and determining other write positions on the basis of write position information of a deflection surface from among a plurality of deflection surfaces.
More specifically, all four different stations S1 to S4 begin writing simultaneously on the basis of respective image signals with reference to a write position detection signal (BD signal) 801 for the station S1 (right end station in FIG. 11).
FIG. 1 is a timing chart illustrating the timing of writing an image after detection of a write position detection signal by a write position detecting means in the color image forming apparatus shown in FIG. 11. As shown in FIG. 12, all the stations S1 to S4 begin writing simultaneously on the basis of respective image signals with reference to a BD signal for the station S1.
The above-mentioned conventional write position detecting methods have the following problems.
In the method (1) above, it is necessary to provide an optical element, a light receiving element (a light receiving sensor), an electric base and the like for each of the plurality of write position detecting means. This results in a complicated apparatus as a whole, causing a higher cost. A unit for controlling rotation of the polygonal deflector is required. This makes it difficult to downsize the apparatus as a whole.
In the method (2) above, a write positional shift (jitter) is produced by a manufacturing error such as a division error of the polygonal mirror deflection surfaces. This causes deterioration of the image quality, and makes it difficult to obtain a highly accurate image. When employing this apparatus as a color image forming apparatus, a difference in write positions in the main scanning direction between colors causes problems such as the occurrence of color shift.